Models, agents, and harnesses for email

Email marketing now runs on three layers: models that generate creative, agents that operate ESPs through API and MCP, and harnesses that connect CRM data to send workflows. Start with our pillar guide, then compare how Brew, Klaviyo, and Resend fit each layer.

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Agent architecture

Models, agents, and harnesses: three ways AI runs email marketing

Language models draft creative. Agents operate ESPs through MCP and API. Harnesses wire CRM data, enrichment, and send logic together. Here is how the layers differ and where Brew, Klaviyo, and Resend fit.

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Agent-operable ESP

Brew review

Email marketing for teams and agents. The harness can call the whole cycle.

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Multi-channel automation

Customer.io review

Event-shaped cognition. The agent reasons about people and triggers.

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Email API

Resend review

Function-level cognition. The agent thinks in send() and events.

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Comparisons

Brew vs Klaviyo: AI-native generation vs ecommerce depth

Brew rebuilds email creative around agents and natural language. Klaviyo owns commerce data and revenue flows.